ANALYTICAL METHODS
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ANALYTICAL METHODS. Lindane was analyzed on a Varian CP3800 Gas Chromatograph with Electron Capture Detection (GC/ECD). Compound separation was on a J&W Scientific DB5 (30m x 0.32mm x 0.25um film thickness). Helium carrier gas, Nitrogen makeup gas – flow rate ~2 mL/min.

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ANALYTICAL METHODS

  • Lindane was analyzed on a Varian CP3800 Gas Chromatograph with Electron Capture Detection (GC/ECD). Compound separation was on a J&W Scientific DB5 (30m x 0.32mm x 0.25um film thickness). Helium carrier gas, Nitrogen makeup gas – flow rate ~2 mL/min.

  • Chloride was analyzed on a Dionex DX600 Ion Chromatography with Suppressed (ASRS Ultra 4mm) Conductivity Detection. IonPac AS17 analytical and AG17 guard columns for compound separation with a flow rate of 1.10 mL/min using a potassium hydroxide (KOH) eluent gradient of 5 mM to 35 mM over 18 min.

Photocatalytic Degradation of Lindanein Potable Water SystemsAmanda M. Nienow*,+, Irene C. Poyer*, Juan Cesar Bezares-Cruz*, Inez Hua*, Chad Jafvert**Civil and Environmental Engineering, Purdue University, West Lafayette, IN 47907+Advanced Concepts and Technologies, International, Waco, TX 76710

ENVR 195

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INTRODUCTION

OH• QUANTIFICATION

EFFECT OF VARYING H2O2 CONCENTRATION

  • Increased terrorist activity in the United States and throughout the world has prompted concern over the security of the nations water sources, purification and distribution systems from possible chemical, biological, radiological, or nuclear (CBRN) and/or toxic industrial chemicals and material (TICs /TIMs) contamination. Technologies, such as reverse osmosis (RO), used in water purification systems for monitoring and providing safe drinking water are effective for most compounds at normal operating conditions, but there are a number of CBRN agents as well as toxic industrial chemicals and materials (TICs and TIMs) that are not effectively removed by reverse osmosis (RO).  The most promising removal technology to use in-line as a replacement for current polishing technologies have been identified and include photochemical processes, such as photocatalytic oxidation (PCO).  These technologies will have the benefit of enhancing performance, reducing the logistical support requirements and potentially enabling continuous polishing treatment of the RO product water, thus reducing the risk of exposure to CBRN, TICS and TIMS.

  • PCO can be broadly divided into direct or indirect photolysis, and homogeneous (single phase - UV/H2O2 or UV/O3) or heterogeneous (two or more phases, e.g., UV/TiO2) systems. Direct photolysis requires target contaminants to possess a chromophore (a functional group on the molecule) which directly absorbs light and reacts. However, molecules without chromophores may participate in secondary photochemical reactions based on their interactions with free-radicals. In the case of H2O2, free-radicals can be generated during photolysis with UV light to produce a highly reactive hydroxyl radical (•OH) :

  • Similarly, O3 can decompose via photolysis or acid-base reactions. The aqueous O3 reaction mechanism varies with pH (more alkaline systems favor ozone decomposition) and as a result produces several different free-radicals:

  • The efficacy of engineered photochemical processes for destroying or transforming chemical agents in a homogeneous system (UV/H2O2 or UV/O3) has been investigated. Preliminary investigative work was completed on the degradation rates of the chlorinated pesticide lindane, one of the most stable toxic industrial chemicals (TICs).

  • Results presented here are for lindane degradation via UV/H2O2 testing effects of:

    • H2O2 concentrations between 0 and 20 mM

    • pH 2.8, 5, 7, 9 or 11.2

    • Suwannee River Humic and Fulvic Acids (IHSS)

Terephthalic acid is commonly used in sonolysis to determine the concentration of OH• [1]. In the presence of the radical, terephthalic acid is transformed into 2-hydroxyterephthalic acid, a compound that fluoresces when excited at 315 nm. Terephthalic acid solutions, with the addition of H2O2, were irradiated and the products were detected with a SLM-Aminco Bowman Series 2 Luminescence Spectrophotometer. The concentration of 2-hydroxyterephthalic acid was then used to determine the OH• concentration.

  • The optimal H2O2 concentration with both 0.26 mM and 13 mM Lindane was between 1 mM and 5 mM, which correlates well with the formation of OH•.

  • The drop in rate constants at higher H2O2 concentrations is likely due to recombination of OH•, also observed in the terephthalic acid experiments (see OH• Quantification Box).

Irradiated solutions with 5 mM H2O2 produced the highest OH• concentrations. The slower rate with higher H2O2 additions is likely due to recombination of OH•.

MONITORING BY-PRODUCTS

TESTING EFFECTS OF pH and NOM

1. pH

* No buffer

  • The fastest photodegradation reaction rates occurred between pH ~5 and pH 7, conditions most closely simulating those of natural groundwater.

  • At pH 9, completed without buffer, the pH dropped throughout the course of the reaction. Due to the change in pH, the observed reaction rate under these conditions is not necessarily first order.

  • At pH 11, Lindane undergoes hydrolysis. However, hydrolysis rate constants are an order of magnitude lower than the rate constants obtained in these experiments, suggesting that the PCO rate constants can be accurately determined by preparing basic solutions of Lindane immediately prior to use. (Note: Upon sitting for several days, hydrolysis products were observed in the basic Lindane solutions).

  • Complete dechlorination of the parent compound was confirmed and quantitated based on the known moles of Lindane and the expected moles of chloride.

  • The formation of an unidentified organic acid was observed during chloride analysis and suggests incomplete carbon mineralization.

  • pH dropped significantly suggesting formation of H+.

  • Additional experiments with longer exposure time are scheduled.

EXPERIMENTAL METHODS

A Rayonet RPR-100 Photochemical Reactor (right) is used to irradiate the aqueous samples. The reactor uses up to 16 lamps with a wavelength of 254 nm. Eight lamps were used in the experiments presented here. The photon flux, determined by chemical actinometry, is 7  10-6 einstein/sec.

A 660 mL quartz tube is placed inside the photochemical reactor. Aqueous solutions of Lindane (~ 0.1 mg/L or 4 mg/L) are added to the tube and irradiated for up to 20 minutes. Some solutions were buffered to pH values of 2.8, 7, or 11.2 with phosphate buffers.

5 mL of solution is removed at a series of reaction times and the contents are either extracted with an organic solvent (for analysis) or sacrificed to measure the pH of the solution. The concentration of the residual parent compound is determined through gas chromatographic analysis. Identification of by-products was accomplished by ion chromatography.

Top View

KEY FINDINGS/CONCLUSIONS

2. Natural Organic Matter (as Humic and Fulvic Acids)

  • Lindane is almost completely mineralized after 45 minutes of irradiation at 254 nm (with a photon flux of 7  10-6 einstein/sec) to form chloride ions and small organic acids.

  • Lindane does not degrade via direct photolysis or by reaction with H2O2 or O3 alone.

  • The optimal conditions for removal of Lindane by UV/H2O2 are a near-neutral pH, ~1 mM H2O2, and minimal amounts of dissolved organic matter.

  • H2O2 photocatalysis is a viable pathway for degrading and removal of organic contaminants from potable water.

REFERENCES/ACKNOWLEDGEMENTS

Rayonet RPR-100 Reactor

References:

[1] Mason, T.J., Lorimer, J.P., Bates, D.M., Zhao, Y. “Dosimetry in sonochemistry: the use of aqueous terephthalate ion as a fluorescence monitor.” Ultrason. Sonochem., 1994, 1(2), S91-94. [2] Larson, R. A., Zepp, R. G., “Reactivity of the carbonate radical with aniline derivatives.” Environ. Tox. Chem., 1988, 7, 265-274.

[3] Haag, W. R., Yao, C. C. D., “Rate constants for the reaction of hydroxyl radicals with several drinking water contaminants.” Environ. Sci. Technol., 1992, 26, 1005-1013.

Acknowledgements:

Advanced Concepts and Technologies, International and TARDEC (U.S. Army Tank Automotive Research, Development and Engineering Center) for funding, and Dr. Changhe Xiao for assisting with organic synthesis and luminescence spectrophotometer analysis.

  • Humic and fulvic acids slow the photodegradation of lindane; at 19.2 mg/L total humic and fulvic acids, the reaction is just slightly faster than the direct photolysis of lindane.

  • Humic acid has a larger effect on the rate constant than fulvic acid.

  • Light attenuation and the scavenging of OH• by the humic and fulvic acids are the major causes of the drop in reaction rate constants. Note: [2] and [3].


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